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Luminous Blue Variables – Characteristics and Definition

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Luminous Blue Variables – Characteristics and Definition LBV and WR nebulae in and beyond our Galaxy Kerstin Weis* & Dominik Bomans Astronomical Institute, Ruhr-University Bochum, Bochum, Germany *Lise-Meitner fellow mail to [email protected] for in- and output Luminous Blue Variables – characteristics and definition LBV ''hot phase'' Main η Car LBV ''cool phase'' Sequence 6.5 With that remark during a talk in 1984, Peter Conti induced the term LBVs more or less ad hoc. He expicility Humphreys Galactic LBVs Davidson excluded main-sequence and Wolf-Rayet star and by chance united the already know classes of S Dor, P Cygni -11 120 LMC LBVs Limit Wray 17-96 M AG Car Pistol and Hubble Sandage Variables into one, the LBV class. This rather vague original definition of LBVs has ⊙ R127 IRAS 18576+0341 changed over time…indeed several characteristics exit, to pin point an LBV ...☺ G79.29+0.46 S61 85 6.0 R143 G26.47+0.02 -10 LBVs are luminous evolved stars that show unique photometric and spectral variabilities ! Sk- 69 279 P Cyg S Dor S119 ⊙ L / l o 60 W243 L b WRA 751 Sher#25 Cool Hypergiants g S Dor variability or S Dor cycle : M G24.73+0.69 o He 3-519 l Within a few years to decades by enlarging and shrinking the radius the spectral type of an LBV HR Car 5.5 -9 HD160529 R110 changes from O-B to A-F and back. Changing to an A-F spectrum causes a increase in the V magnitude R71 (typically below 1mag) and a redder B-V color. In one cycle the star moves accross the HRD from a ''hot'' to a 40 HD168625 Red ''cool'' and back to the ''hot'' phase (Fig. 1). Doing so the stellar wind changes from a fast, low density wind (hot) Super SN 1987A Giants -8 x to a slower optically thick wind (cool). Variations of the wind density and velocity gives rise to wind-wind 5.0 interactions and can lead to the formation of circumstellar LBV nebulae. 4.8 4.7 4.6 4.5 4.4 4.3 4.2 4.1 4.0 3.9 3.8 3.7 3.6 3.5 log Teff Giant eruption: Fig1.: HRD with hot and cool phase position of Galactic and LMC LBVs. Significantly larger photometric variations occur during a giant eruptions outburst. The brightness of an LBV The main- sequence, RSG and cool hypergiant regions are plotted as increases spontaneously by several magnitudes and larger amounts of mass are ejected within a few years. reference. Figure adapted from Weis & Duschl (2002) LBV giant eruptions have been mistaken for supernovae (e.g. SN1954J) ! ck qua With that remark during a talk in 1997, Bruce Bohannan stated that LBVs can be distinct from other hot or cold massive stars, by seeing it quack. The quack being an S Dor cycle or giant eruption. Evolutionary state: LBVs are evolved massive stars in transition from the main sequence to Wolf-Rayet stars. Observations and theoretical stellar evolution models which include rotation (Maeder et al. 2005) find Tab.1: Taken from Maeder et al. (2005), this tables illustrates the various ways how stars may evolve and enter the LBV and WR phase. LBVs to stars with an initial mass as low as Mini ~ 22 M⊙ up to 120 M⊙. LBV nebulae … bipolar jewels Circumstellar LBV nebulae are created either by the wind-wind interaction of faster and slower wind phases during the LBV phase or via the ejection of outer layer of the star during a giant eruption. All LBV nebulae show a stronger [N II] emission, as CNO processed material is mixed up and peeled off by stellar winds or in a giant eruption. Fig. 3 shows the images of the galactic and LMC LBV nebulae, their basic values are listed in Tab. 2. Bipolar morphology appears either in a typical hourglass shape like the Homunculus around η Car (Fig. 2) or as bipolar attachments (caps) (e.g. R 127, Fig. 3). η Car Galactic LBV nebulae (excluding η Car) LMC LBV nebulae • Size: • Size: 0.15-2 pc, average ∼ 1 pc 0.82-6.2 pc, average ∼ 2.1 pc • Morphology: • Morphology: 33% spherical/elliptical 50% spherical/elliptical 0% irregular 25% irregular 67% bipolar 25% bipolar Fig.3: Galactic and LMC Tab.2: Sizes, expansion velocities, • Expansion velocities: • Expansion velocities: ↑ ↓ nebulae on scale ages and morphologies of LBV nebulae AG Car 25-150 km/s 12-27 km/s Fig.2: HST images Galactic versus LMC LBV nebulae: Comparing Galactic and LMC nebulae indicates that of two bipolar LBV LMC nebulae are generally larger and expand slower. The fraction of bipolar nebulae is nebulae (from Weis larger among galactic LBVs, in the LMC so far only shows R127 bipolarity. 1999, 2011). Why are there many bipolar LBV nebulae ? Looking at the morphology of LBV nebulae it is obvious that a significant fraction of LBVs are bipolar, for the galactic LBVs its as high as 67%. One clue for that high rate of bipolarity comes from that fact that LBVs can show a large rotation. Large rotational velocities are reported for AG Car (Groh et al. 2006) and HR Car (Groh et al. 2009), both with bipolar nebulae. Strong mass loss, the stars proximity to stability limits, either Eddington or ΩΓ-limit, and a high rotation yields ideal conditions to form and favor bipolar nebulae !!! (tabel & figure Weis 2012) LBV nebulae … what JWST can do? LBVs are known in many other local group galaxies and as far out as M101 (about 7 Mpc). A large sample of LBVs (the former Hubble-Sandage Variables) are in M33 and M31, the only other spiral galaxies in the Local Group host LBV stars . With distances of 850 kpc (M33) and 773 kpc (M31) LBV nebulae, assuming similar sizes as in the Galaxy and LMC, are ½ to 1'' large. So a direct search for nebulae is not possible, but we find hints for nebular emission from [NII] lines (5755, 6543, 6583Å) in MMT spectra and IFU data from different telescopes. Even short observations with the JWST/NIRCAM however would give way to direct imaging of these nebulae.Using Pα (F187N) images LBV nebulae with a size of 0.15'' can be resolved, maybe even smaller with an adeqate PSF substraction. Assuming a typical radius of 2pc LBV nebula could be detected in galaxies out to a distance 1.5 to 4 Mpc. Most important will be a rough estimate of the morphology of the nebulae. Is the large fraction of bipolar nebulae also Fig.4: M33 host of many LBVs and LBV candidates. present in other galaxies, or not. The numbers for the galactic and LMC bipolar nebulae show a lower value among (figure Burggraf et al. 2014) the LMC, but with a rather small number statistics it is to early to draw a good conclusion, whether or not a lower metalicity could play a role. Observations of LBV nebulae with the JWST in galaxies beyond the LMC will significantly change our understanding of the LBV phase and the formation of LBV nebulae. It will contribute to the identification of what physical mechnism causes the S Dor variability and which instabilities drives the really energetic the giant eruption ! .
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